Exclusive: Octopus has never been more popular on menus. Yet overfishing threatens supply. How one band of entrepreneurs and scientists is braving activist-filled waters to establish the world’s first commercial octopus farm. Balancing on a railroad-tie-size beam of a platform floating in Spain’s Vigo Bay, Ricardo Tur crouches and points below. Dangling several feet underwater is a pen the size of a garden shed, home to 80 octopuses.